Zinc diammine chloride (Zn(NH3)2Cl2, CAS #14826-66-5, a.k.a. zinc ammonia complex, diamino zinc dichloride, zinc diammine dichloride) (ZDC) has been primarily known as an intermediate for production of zinc chloride. It is not commonly known as a commercial product.
Crystallization of zinc diammine chloride from aqueous solutions has been a known technology for many years. Crystallization has been used as a method of separating and concentrating zinc from other cations, especially iron and heavy metals. A number of patents have been issued using this technology to recover zinc values from various by-product materials, such as steel mill dusts and other zinc-containing materials. Many of the patents use a process in which the zinc fume is collected in a scrubbing system containing an ammonium chloride solution. The zinc diammine chloride is crystallized from the ammonium chloride solution by adjusting the pH of the solution to near neutral.
Zinc diammine chloride can be produced in crystalline form. Zinc diammine chloride crystallizes as an unhydrated compound and can be grown into long needle-like crystals. The surface moisture can be easily dried by use of any typical material drying apparatus or process. Patents have been issued concerning the production of anhydrous zinc chloride from zinc diammine chloride crystals. In these patents, the material is heated to decomposition to drive off ammonia.
Agricultural
Zinc deficiency in soil is a common cause of poor plant growth. Zinc deficiencies can be remedied by applying a zinc salt to the soil directly or in an admixture with other plant nutrients.
Aqueous solutions of zinc ammine chloride and zinc ammine sulfate have been manufactured and marketed for years. These products are typically zinc tetraammine complexes and have been used as a source of zinc. There have been several patents covering the manufacture and use of zinc and other micronutrient ammine complexes as aqueous solutions. The patents cover aqueous zinc ammine complexes with both inorganic and organic anions, the most common being sulfates, chlorides, and nitrates. Some of the other notable use patents cover the manufacture of zinc ammine acetate or other organic anions and its use in anhydrous ammonia. The aqueous zinc ammine complexes are stable enough to prevent precipitation of zinc compounds when added to liquid fertilizers, such as urea/ammonium nitrate and polyphosphate fertilizers. The fertilizer market for such liquid zinc ammine complexes is mature and limited.
Zinc diammine chloride could also be used as a zinc raw material in liquid fertilizers. Zinc diammine chloride does not, however, dissolve as rapidly or mix as easily as the aqueous tetraammine complex. Dry zinc diammine chloride would be more economical to transport since the zinc content is higher. The zinc content of pure zinc diammine chloride is 38.4%.
The majority of zinc applied as fertilizer is used as a blend with other dry fertilizer materials. The most common sources of dry zinc for fertilizer are zinc sulfate (36% Zn) or zinc oxy-sulfate (20% Zn). These products are marketed as granules with the optimal size being matched with the particle size of the fertilizer material to prevent segregation. Some dry zinc fertilizer products, especially the zinc oxy-sulfates, contain high levels of iron and heavy metals. Environmental concerns are driving the market towards purified zinc sources with low heavy metal content. Zinc diammine chloride can solve this need by having very low heavy metal content and provide both zinc and nitrogen as sources of plant nutrients.
In order to produce particles of sufficient size to use the ZDC as a fertilizer, a need exists for a method to produce granules which are consistent in size, hard and nonfriable and which does not drive off an excess amount of ammonia resulting in zinc chloride. What constitutes a xe2x80x9csufficient sizexe2x80x9d is generally thought to be that which meets uniformity index (UI) and size guide numbers (SGN) for blending with other fertilizer components. For example, The Fertilizer Institute published a Bulk Blend Quality Control Manual in September 1987 which is designed to help producers and blenders in matching (which eliminates segregation) and compatibility of materials. Blended fertilizers are generally mixtures of granules of xe2x88x926+16 mesh, but the most important consideration is matching the other components in the mixture. See e.g., Cecil Russell, Quality of a Blend Can be Determined, Farm Chemicals, March 1965, p. 86; George Hoffmeister, et al. Bulk Blending of Fertilizer Material: Effect of Size, Shape, and Density on Segregation, Agricultural and Food Chemistry, 1964, p. 64. Alternatively, crystallized zinc diammine chloride can be made into aqueous solution for fertilizer use at distant locations to lower the cost of transportation.
Adjustment of Galvanizing Preflux
There are many processes which produce zinc-containing by-products or waste products which can be utilized as a raw material for production of zinc complexes which have value. One such process is galvanizing. Galvanizing produces xe2x80x9csal ammoniac skimmings, sal skimmings, or wet skimsxe2x80x9d. Many galvanizers use a xe2x80x9ctop fluxxe2x80x9d composed of zinc chloride/ammonium chloride double salt in what is called a xe2x80x9cwet kettle processxe2x80x9d. The ratio of zinc chloride to ammonium chloride for a galvanizer""s top flux can vary depending on the particular application. The most common ratio is a double salt containing 1 mole of zinc chloride to 3 moles of ammonium chloride. The metal to be galvanized is pickled in either a dilute hydrochloric acid or sulfuric acid solution to clean and prepare the surface, and then it is dipped into a vat of molten zinc through a layer of xe2x80x9ctop fluxxe2x80x9d. The flux is floated as a melt on top of a vat of molten zinc and the pieces to be galvanized are passed through this layer of flux to aid deposition of zinc. Over time, much of the ammonium chloride portion of the flux is sublimed and the flux becomes contaminated with zinc oxide, charred organics, and various other impurities. When the flux is no longer effective, it is skimmed off and sold as a by-product to various industries as a source of recyclable zinc, i.e., the xe2x80x9csal ammoniac skimmingsxe2x80x9d, xe2x80x9csal skimmingsxe2x80x9d, or xe2x80x9cwet skimsxe2x80x9d.
Other galvanizers use a prefluxing process in which the zinc chloride/ammonium chloride double salt is dissolved in water and the pieces to be galvanized are dipped into the flux solution and then into a vat of molten zinc. The zinc chloride/ammonium chloride ratio in these preflux solutions varies depending upon the application. Most operations use ratios of either 1:3 or 1:4 zinc chloride/ammonium chloride. This type of galvanizing is known as xe2x80x9cdry kettlexe2x80x9d. Over time, oxidation of the molten zinc occurs and a layer of zinc, iron, and other metal oxides form on the surface of the molten zinc. This layer is also skimmed off to prevent problems in galvanizing and is commonly known as xe2x80x9cdry zinc skimsxe2x80x9d. It is composed of the various metal oxides as well as metallic zinc. This material is more easily recycled than the xe2x80x9cwet skimsxe2x80x9d and commands a higher price.
Dry galvanizing also goes through a pickling process to clean and prepare the metal surface. Unless the metal is well rinsed, carryover from the acid-pickling tank gradually contaminates the preflux solution and lowers the pH to a point where it no longer performs properly. At this point the flux solution must either be changed or adjusted chemically. Properly maintained preflux solution has a pH of 4.0-4.5. One option for adjusting the solution pH is to add some of the xe2x80x9cdry skimsxe2x80x9d from the zinc kettle. The zinc oxide reacts with the excess acid to form zinc chloride. This option changes the optimum zinc chloride/ammonium chloride ratio and requires the addition of ammonium chloride to maintain the balance. Another option is to add ammonia either as anhydrous or aqueous solution. This also changes the optimum zinc chloride/ammonium chloride ratio and zinc chloride needs to be added to maintain the balance.
A better option for adjustment of preflux tanks is needed. Ideally, this would be one which adjusts the pH while maintaining the correct ratio of zinc and ammonia. Zinc skims are primarily ZnO. The addition of ZDC to a preflux solution to adjust the pH is better than current methods because bags of material are easier to store, measure, and use; the ZDC addition is more accurate since ammonia is difficult to meter, and zinc skims have viable analysis and are more difficult to handle; and the addition is much cleaner due to the absence of contaminants found in zinc skims, such as those that come from steel work and the lead content (over 1%).
An object of the invention is a method for separating zinc diammine chloride from a mixture of zinc ammine chloride and zinc ammine sulfate.
Another object of the invention is a method of granulating zinc diammine chloride.
A further object of the invention is a method for using zinc diammine chloride in or as a dry fertilizer.
Another object of the invention is a method for using zinc diammine chloride for preparing zinc ammine chloride solutions.
Yet another object of the invention is a method of using zinc diammine chloride as a galvanizing preflux adjuster.
These and other objects, features, and advantages will become apparent after review of the following description and claims of the invention which follow.
Zinc diammine chloride (ZDC) can be produced by preferentially precipitating zinc diammine chloride from a mixture of zinc ammine sulfate and zinc ammine chloride. The method for preferentially precipitating ZDC is to lower the pH of the solution to near neutral by addition of an acid, hydrochloric acid. The precipitated ZDC has little, if any, sulfate present.
For some applications of dry ZDC, the xe2x80x9cnaturalxe2x80x9d crystalline form of ZDC is inadequate. Granules are often a more desired form. Melt granulation can be an effective method of granulating ZDC. Compaction granulation can also be used effectively for granulating ZDC. Melt granulation is achieved by heating crystalline ZDC to between about 340xc2x0 F. and about 360xc2x0 F. to achieve the desired size granules. Compaction granulation of ZDC can be performed using conventional compaction granulation techniques and, optionally, preheating to aid in the granulation. The methods of granulating ZDC are able to achieve granulation without substantial loss of ammonia.
ZDC can be used as a zinc source in a dry fertilizer. The use of ZDC in a dry form has advantages over other zinc compounds which cause handling problems in a dry form and over aqueous solutions. In addition to the handling problems of other zinc compounds, ZDC can provide zinc without contamination by other metals. ZDC crystals or granules are added to dry fertilizer formulations in an amount desired to achieve the appropriate level of zinc. ZDC also provides a nitrogen source. The nitrogen level of the remainder of the formulation takes into account the nitrogen provided by the ZDC.
The present invention also provides a method of using ZDC as a galvanizing preflux adjuster. Addition of ZDC until the desired pH is reached will not only adjust the pH but will assist in maintaining the correct ratio of zinc chloride and ammonia chloride. Another advantage of using ZDC as the preflux adjuster is that ZDC can be produced by use of the galvanizing waste. Ammonium chloride can be premixed with the ZDC in order to further maintain the proper ratio of zinc and ammonia, even if some supplemental adjustment is needed of the ratio.
Though ZDC can be made from any zinc source, spent galvanizing fluxes provide an xe2x80x9cidealxe2x80x9d source of raw material to produce ZDC. These spent or contaminated fluxes contain impurities such as iron, sulfates, and lead. The pH of the solution is raised to approximately 10 with anhydrous ammonia to complex the zinc in solution. The impurities, such as iron and lead, are then separated from the solution to yield a purified solution of zinc ammonium chloride with traces of sulfate.
It has been found that adjusting pH to near neutral is an effective method of separating chlorides from sulfates in a mixed ammoniacal solution containing zinc ammine chlorides and zinc ammine sulfates. The optimum pH for separation varies depending on what else is in the solution. At a neutral pH, the zinc diammine sulfate is more soluble than the zinc ammine chloride. Zinc diammine chloride can be produced by preferentially precipitating the zinc diammine chloride from a mixture of zinc ammine sulfate and zinc ammine chloride by lowering the pH to near neutral by addition of an acid, hydrochloric acid. The hydrochloric acid provides part of the chloride ion to the ZDC. Though other pH adjusters can be used, they do not appear to be as efficient. For example, if sulfuric acid is used, the zinc is more soluble as a zinc ammine sulfate and the separation efficiency drops off. By simply neutralizing the solution, the zinc diammine chloride crystals will contain some sulfate (about 10% Of the original amount or lower). It is expected that the level of sulfate can be reduced to very low levels by sequentially washing the crystals or going through a recrystallization process. The ZDC can be ammoniated and diluted to form a liquid zinc ammine complex. Then the pH can be adjusted again to a neutral pH using HCl. A large portion of the remaining zinc ammine sulfate will stay in solution. This process may also be effective for separating chlorides from anions other than sulfate, nitrates, for example. Nitrates are more soluble than chlorides at a neutral pH.
Essentially all the sulfate present in a zinc ammine sulfate/chloride mixture remains in the filtrate when zinc diammine chloride is precipitated by lowering the pH to near neutral by the addition of hydrochloric acid. Generally, metal chlorides are more soluble than metal sulfates (zinc chloride solutions contain 30% zinc at ambient temperatures where as zinc sulfate contains 12% at ambient temperatures) (see Perry""s Chemical Engineers"" Handbook, 7th Ed., 1997, Robert H. Perry and Don W. Green, p. 2-124; CRC Handbook of Chemistry and Physics, 75th Ed., David R. Lide, Editor, 1994, Sections 4-112-113; CRC Handbook of Chemistry and Physics, 31st Ed., Charles D. Hodgman, Editor, 1949, Chemical Rubber Publishing Company, pp. 546-557), therefore, it was unexpected that Zn(NH3)2Cl2 would precipitate in preference to Zn(NH3)2SO4. Generally, solutions of zinc ammine complexes can be manufactured which contain 20% zinc or more. The highest zinc analysis practical using sulfate as the anion, due to salting out (surpassing the solubility limit), has been around 15% zinc. To produce a 20% zinc solution with a low salt-out temperature, chloride has been used as the primary anion. These solutions would have a salt out temperature of around 10xc2x0 F. The 10xc2x0 F. salt out temperature is desirable for storage and handling for example in the northern Corn Belt where much of this type of fertilizer is used.
When zinc diammine chloride is made from an aqueous ammoniacal solution containing 15% zinc by the addition of hydrochloric acid, it is soluble at a temperature of around 200xc2x0 F. The material should be cooled to below 100xc2x0 F. to crystallize out most of the ZDC, depending on the concentration used since the temperature is roughly proportional to the concentration.
Alternatively, it is believed the pH can be raised on a zinc chloride solution to produce ZDC as well. Zinc diammine chloride has not been previously produced as a granular material. The present invention provides methods for producing granulated zinc diammine chloride.
Zinc diammine chloride crystallizes as an unhydrated compound and can be grown into long needle-like crystals. The surface moisture can be easily dried by use of any typical material drying apparatus or process. Anhydrous zinc chloride has been produced from zinc diammine chloride by heating to decomposition of the ZDC which drives off the ammonia. The present method is able to granulate the ZDC without significant loss of the ammonia and conversion to zinc chloride.
The present method heats the ZDC at temperatures substantially above those required to evaporate the surface moisture but below the decomposition temperature. This controlled heating will melt the ZDC and allows for granulation without substantial loss of ammonia. The literature value listed for the melting point of ZDC is 210.8xc2x0 C. (410xc2x0 F.) (CRC Handbook of Chemistry and Physics, 75th Ed., David R. Lide, Editor, 1994, Section 4-113). The decomposition temperature is 271xc2x0 C. (520xc2x0 F.) (CRC Handbook of Chemistry and Physics, 75th Ed., David R. Lide, Editor, 1994, Section 4-113). Experimental data shows that ZDC can be granulated at temperatures between 340 and 360xc2x0 F. The temperature range expected to be useful is about 320xc2x0 F. to about 380xc2x0 F. This is substantially below the listed melting point for ZDC. It may be that the presence of the relatively small amount of ammonium chloride present forms a eutectic mixture with zinc chloride generated by the loss of ammonia and acts as a binding agent for the remaining ZDC. The material produced has very low hydroscopicity which would indicate that little zinc chloride is present. A granulated ZDC would have many applications, such as fertilizer or use in flux adjustment as described below.
For spherical particles, it is believed that rolling the ZDC while heating adequately granulates the ZDC.
An alternative to melt granulation of the ZDC is compaction granulation. Since the compactor creates heat, some of the ZDC may reach melt temperature and form granules. Compaction granulated ZDC could be ground and sized to match particle sizes of other solids with which it might be mixed. Preheating the material before compaction may aid in the compaction granulation process. Conventional compaction granulation techniques would be used. One of skill in the art would readily be able to determine what process conditions to use in order to achieve granulated ZDC with desired properties. ZDC granulated by this method would have the same sorts of applications as those of melt granulated ZDC. Since ZDC has low water solubility, it is expected to be as difficult to compaction granulate as table salt, therefore, since the ZDC is not sticky, it may require higher pressures to compact.
Fertilizer
A more economical way to produce and distribute a zinc source of zinc diammine chloride for use, such as in fertilizers, is in a dry form. The dry form of zinc diammine chloride is able to provide a zinc content of up to 38.4%, higher than other zinc sources and without contamination of iron or heavy metals. In addition to providing a source of zinc, the use of zinc diammine chloride would provide a nitrogen source in the fertilizer formulation. One of skill in the art would readily be able to determine the amount of ZDC to add to a fertilizer formulation in order to provide the desired level of zinc and nitrogen. ZDC has not been commercially available as a dry fertilizer.
Another use for ZDC is preparation of zinc ammine chloride solutions. The high zinc analysis of ZDC would perhaps reduce freight costs to distant locations due to its higher zinc content. The ZDC can be dissolved with water and ammonia to form a highly soluble zinc ammine chloride solution. One of skill in the art would be able to determine mixing order and ratios of the components desired for a particular application of the end solution.
Galvanizing Flux Adjuster
Galvanizing by-products can be used as a starting material for zinc ammine complexes. The zinc diammine chloride can be used back in the galvanizing process as well. Zinc diammine chloride can be used as an effective pH adjusting material for preflux tank adjustment. The zinc diammine chloride would be added to the preflux tank in an amount sufficient to bring the pH to the desired range of 4.0-4.5, when it drops below this during use. Zinc diammine chloride is better than adding xe2x80x9cdry skimsxe2x80x9d or ammonia since either of these adjusters change the zinc chloride/ammonium chloride ratio which requires also adding the other portion of the double salt in order to maintain the balance. The most commonly used molar ratios of zinc chloride/ammonium chloride in preflux solution is either 1:3 or 1:4. Zinc diammine chloride is odorless and much safer to use and store than anhydrous ammonia. The zinc/ammonia ratio of zinc diammine chloride is 1:2. Zinc diammine chloride crystals have experimentally been created which contain about 5% ammonium chloride. Addition of these crystals would tend to gradually change the zinc chloride/ammonium chloride ratio but not as much as the addition of either xe2x80x9cdry skimsxe2x80x9d alone or ammoniation. Zinc ammine chloride can be added by itself to adjust the pH or can be premixed with an amount of ammonium chloride to further maintain the proper ratio.